1. Field of the Invention
This invention relates to improved fibrous sheet material, and methods and apparatus for forming it, and more particularly, to fibrous sheet material which is patterned bonded and differentially creped to impart thereto a combination of improved tensile strength, softness, bulk and substantial stretch in all directions in its own plane.
2. Description of the Prior Art
In the past, there has been extensive activity in the field of papermaking to discover ways of imparting softness to paper webs without degrading their strength. Paper webs are conventionally softened by working them in different ways, such as by creping them from a drying surface with a creping blade. Such a process disrupts and breaks many of the interfiber bonds in the paper web which are formed during the drying thereof by the hydrate bonding process associated with papermaking. However, these interfiber bonds are the principal source of strength in an ordinary paper web. Very little strength results from the physical entanglement of the fibers since papermaking fibers have such an extremely short length, generally on the order of 1/16 inch or less.
Attempts to improve this situation have involved the creping of webs in only a selected spaced-apart areas over its surface, such as by creping with a notched or serrated creping blade, or creping from a discontinuous surface such as a circumferentially grooved roll, leaving the portions therebetween with substantially all of their strength. However, such creping patterns necessarily created lines of weakness through the sheet so that the ultimate sheet was not very strong at least in certain directions.
One of the characteristics of a sheet product which gives the semblance of strength is the toughness of the sheet. In essence, this is representative of a combination of the tensile strength of the sheet and the ability of the sheet to stretch. Obviously, if the sheet can absorb some work imposed upon it by stretching so as to avoid firmly resisting the full force applied, the resulting web appears subjectively to be stronger. It has long been known to crepe webs in various ways to create stretch and, accordingly, to impart toughness. However, even webs which have been creped in one direction, or in several different directions so as to impart universal or isotropic stretch, are weakened by the creping, and accordingly, do not have as much strength as desirable.
In the field of nonwoven webs, which generally include substantial amounts of fibers having a length greater than 1/4 inch, it has been common practice to apply bonding material to spaced portions of the web so that fibers in at least portions and perhaps in a network across the web become bonded together to impart strength to the web. However, the fibers in such nonwoven webs are sufficiently long to enable small amounts of adhesive to impart substantial strength to the web since any two adjacent areas of adhesive application can be quite far apart and yet be able to bond one fiber into a network.
It has often been thought that to apply bonding material to a paper web to impart strength thereto would result in harsh areas in the sheet which would destroy any feeling of softness which is desirable. In addition, in view of the extremely short length of papermaking fibers, it has been felt that the amount of bonding material and the large percentage of the overall area of the sheet which would have to be impregnated to impart any strength to the sheet would result in a very hard sheet, having little or no stretch and poor softness characteristics.
It was therefore quite unexpected and surprising to discover a method of applying a bonding material to a paper web to impart strength thereto without impairing the softness thereof and, furthermore, to increase the bulk of such a web and to impart substantial stretch in all directions in the plane of the web. This method enables even softer and bulkier webs to be formed and utilized than was heretofore possible in view of the addition of substantial strength to the web by the bonding material. Thus, webs could be formed in a conventional manner on a papermaking machine from fibers which were treated with a chemical debonder to reduce the interfiber bonding capacity thereof, or under conditions of reduced pressing during web formation to reduce the amount of interfiber bonding in the web, or by treating a web which had been previously creped and thereby weakened, all of such webs being characterized by very little interfiber bonding strength. However, by the method of this invention, this deficiency is overcome by the application of a bonding material thereto in a fine pattern. In addition, the method of this invention involves the creping of the bonded areas of the web which results in the softening of the surface thereof to remove the harshness previously experienced due to bonded web portions. Unexpectedly it was discovered that such creping not only softened the surface of the bonded areas, but also generally substantially increased the bulk of the web. In addition, such creping shortened only such bonded portions of the web in a manner causing buckling or puffing of unbonded areas of the web so that substantial stretch in all directions in the plane of the web was achieved.